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VGB PowerTech 4 l 2018 New hydroelectric facility in Romanche-Gavet

Authors

Kurzfassung

Die neue Wasserkraftanlage in Romanche-Gavet

Eine neue Wasserkraftanlage bei Romanche Gavet, die derzeit am Fluss Romanche, etwas außerhalb des Oisans-Gebietes, gebaut wird, ersetzt sechs alte Anlagen. Die Besonderheit die-ser Laufwasserkraftanlage besteht in einer sehr begrenzten Aufstauung und einer sehr langen kurzgeschlossenen Strecke. Um die Wasser-kraftnutzung des Flusses zu erschließen, hat sich EDF zudem für eine beispiellose Technolo-gie in Frankreich entschieden: Energiedissipa-toren. l

New hydroelectric facility in Romanche-Gavet, FranceEnergy dissipators: innovative technologyPhilippe Llusia

Philippe LlusiaProjet Romange GavetEDF - Centre d’Ingenierie Hydraulique

New hydroelectric facility in Romanche-Gavet

A new facility at Romanche Gavet, current-ly under construction on the Romanche river, just outside the Oisans area, replaces six old power plants. The specific feature of  this run-of-river facility is to have a very  limited impoundment and very long short-circuited stretch. To secure access to  the river, EDF has opted for unprece-dented technology in France: energy dissi-pators.

A new hydroelectric facility

This new facility will replace five dams and six old power plants on the Middle Roman-che river. Most of these plants, in operation during the construction of the new facility, go back almost a hundred years. After stud-ies conducted by the Hydroelectric Engi-neering Centre (CIH) of EDF, it was decid-ed to replace them by one single, more ef-ficient facility to optimise the head flow. (See F i g u r e 1 )This new facility, consisting of a dam, a 9.3 km long headrace, an underground plant and a few external buildings, operates the same stretch of river as all the current six plants put together. More efficient, it is

used to produce 35% more energy with the same head flow. This production gain cor-responds to the average consumption of a town with a population of 60,000.More environmentally-friendly, it clears the landscape of old equipment (plants, dams, penstocks, inlets, channels, etc.), it restores ecological continuity by removing river obstacles such as dams and uses plant engineering techniques to ensure the sta-bility of the banks.

Sharing uses of water and securing the short-circuited stretch

The construction of a new facility offers the opportunity to factor in other uses of water (fishing, tourism, white water sports, etc.) and improve river safety. For that, it is nec-essary to raise the prefectural order cur-rently in force, which prevents access to the short-circuited stretch of the Romanche. On the old facilities, there are no systems to delay the opening of the dams when a turbine does an emergency shutdown. This shutdown causes major flow variations in the river and can generate a risk for people present in the bed of the river.

Fig. 1. Diagram of the facility as a whole. Upstream, the Livet dam can impound part of the water of the Romanche in the headrace (dotted line). This 9.3 km headrace is built by two tunnel boring machines from the Les Ponants adit and links the dam to the Gavet plant located downstream. Excess water, or at least the instream flow, is poured into the short-circuited stretch (in blue), a natural stretch of the river between the dam and the plant.

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New hydroelectric facility in Romanche-Gavet VGB PowerTech 4 l 2018

Dissipating to ensure a time delay

On the new facility, the dam capacity is not enough to store all the upstream flow in the case of a sudden shutdown of the turbines. As the flows impounded can reach 50 m3/s, it is no longer possible to open the valves of the dam to immediately release non-tur-bined water without putting people in the bed of the river at risk. Such a flow would cause extremely strong and excessively rapid rising waters. It must therefore be possible to perform a warning release to notify anyone present in the river of an im-pending rise of the waters. Moreover, the short-circuited stretch is long (approxi-mately 10 km) and it is important to con-sider the time to propagate this warning release. To delay the opening of the dam gates while performing this warning re-lease, it is therefore necessary to dissipate the value of the flow crossing the turbines when these are shut down. This warning release, agreed between EDF and the State as part of the licence agreement, consists of releasing only 10 m3/s for a duration of 40  minutes. This flow is added to the in-stream flow which is 3.83 m3/s. After these 40 minutes, EDF can then switch the entire river flow into the short-circuited stretch.

An innovative technical choice

It has therefore been decided to continue to impound the water in the dam to lead it downstream, at the level of the plant. There, a dedicated penstock, connected to vertical armoured wells, offers a way to by-pass the plant and its turbines and lead the water taken from the river to the energy dissipators located on the plant’s restora-tion platform. They will dissipate the hy-draulic energy instead of the turbine. This system is exclusively used in the case of emergency shutdowns. For scheduled shutdowns of the turbines, the warning re-lease at the dam is programmed simultane-ously with the decline of turbined flows. Finally, the dissipators have an operating time that corresponds to the time needed to perform the warning release.Conventional energy dissipation systems usually rely on cone jet technology. This could not be applied at Gavet as it would have required space that is not available given its location at the foot of a cliff on a very narrow natural platform. The techni-cal underground dissipation technique was also envisaged but it would require exces-sive excavation work. A multi-jet dissipa-tion system was therefore finally chosen for Romanche Gavet. This technology, although tried and tested since the 1990s in Italy at a single site, is innovative as it has experienced an inter-esting technical breakthrough since 2009 under the impetus of D2FC Energy Valves who design and manufacture this equip-

ment. It is currently found in three Cana-dian facilities, in Albania, Columbian, Jor-danian plants and soon, in France, at Ro-manche Gavet. This technical choice is therefore both innovative and linked to the geographic context of the new facility.

High performance equipment

Each dissipator can dissipate 0 to 25 MW. There are four dissipators, making it pos-sible to dissipate all the power of the flow impounded at the dam. A dissipator weighs 30 tonnes, measures 8 m tall x 1.20 m in di-ameter and is placed at the bottom of a 6 m diameter x 7 m tall pit (see F i g u r e   2 ). A barrel valve located at its lower end is acti-vated by a hydraulic valve and releases wa-ter through submerged nozzles at the bot-tom of the inundated pit. Dissipation occurs by combining back-pres-sure (position at the bottom of the pit) and a physical phenomenon called Von Karman vortices, i.e. a flow in alternated, energy-consuming vortices. When a dissipator op-erates at full flow, the water overflows into the pits in a one-meter high wave, then joins the river. At the bottom of the dissi-pating pits, a second pit, called the damp-ing pit, closed by a 1.20 m high spillway wall, helps to absorb the shock once the water has been restored to the river. The pit into which the dissipators release water are fitted with a 3 m high shield at their base, corresponding to the active part of the dissipator (see F i g u r e 3 ). Each dissipator has a security system con-sisting of a 900 mm ball valve. These valves are connected to a manifold, itself linked to the dissipators’ penstock. All this equip-ment is sized to resist Maximum Instant Operating Pressure which, at Gavet, is equal to a 347 m water column, equivalent to 34.7 bars where the operating pressure is 27 bars.

Downstream from the ball valve, an offtake (conical element of the penstock) reduces the 900 m diameter to 120 mm of the body of dissipators while reducing the speed of the water flow.

Reactive equipment

In case of an emergency shutdown of one or several turbine generators, the flow is cut off at the turbine by closing the distrib-utor that supplies the turbine wheel with water, then by closing the ball valve sup-plying the generator. At the same time as this shutdown, one or several ball valves on the dissipator are opened simultaneously

Fig. 2. Overview of the dissipator in the study phase using 3D software. (© D2FC)

Fig. 3. Installed in September 2015, elements of the armouring of the dissipator pits were first welded then sealed to the concrete of the structure. (© C. HURET)

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VGB PowerTech 4 l 2018 New hydroelectric facility in Romanche-Gavet

followed by the opening of the dissipators themselves for a value equal to the flow run by the turbines of the generator or genera-tors during shutdown. All this equipment is steered by a control system consisting of PLCs that ensure real-time steering.In the event of an emergency shutdown of the generators linked to a mains fault, the hydraulic plant that activates the dissipator valve remains operational, as the energy needed to move it is stored in the oleo-hy-draulic accumulators’ battery.

Closely-monitored production

D2FC Energy Valves, which designs, manu-factures and installs this equipment is a French company headquartered near Fé-

camp in Normandy. The construction ma-terials come from German and Italian steel plants but are made and assembled in France. The pits and machine tools are made in the Grenoble area, at Pont de Claix and the hydraulic plants in Toulouse. A subsidiary of the company, D2FC Services, located near Grenoble at Fontaine, assem-bles the machinery on site.

The production of such equipment, on which the safety of the facility depends, undergoes a large number of tests and in-spections. Tests are also performed by the company which has its own quality engi-neer but also under the supervision of EDF materials inspectors and outside auditing organisations. Inspections are conducted on the basic materials to guarantee the

quality of steel and welding during assem-bly. Pressure tests are conducted to ensure perfect watertightness and pressure-resist-ance of the various parts. Finally, the anti-corrosion paint coating is tested to guaran-tee sustainability.

The different assembly phases

The assembly of the dissipators is part of the phasing of the construction of the building housing them. It was firstly neces-sary to build the foundations to install the pits. Their armoured reinforcements, de-livered in two parts in autumn 2015, were firstly welded then sealed to the concrete of the structure. Their installation required very precise altimetric mapping and hori-zontal positioning as the whole system then needed to be linked to the penstock installed once the armoured wells were completed. The pits were concreted at the same time as the elevation of the building (33 m long x 18 m x 18 m h) between late 2015 and early 2016. To concrete the pits without affecting their roundness, six batches of concrete (50 cm x 3 m h) were needed as stress on the reinforcements was high. The ar-moured tanks were also solidly supported internally and six external removable stiff-ening rings (hoops) were used to maintain the geometry of the pits. In May 2016, the manifold (see F i g u r e  4 ) was installed on its supporting concrete blocks and pillars. Then, in March 2017,

Fig. 4. Installation of offtakes in March 2017 Located between the ball valve and the dissipator, they are slightly conical shaped and help to slow down the water flow. (© C. HURET)

Fig. 5. Installation of the manifold on 6 May 2016. It will distribute water from the penstock dedicated to the dissipators and connected to the armoured wells, on the four ball valves of the four dissipators. © C. HURET

Fig. 6. The first two dissipators were delivered mid-September 2017 and the two oth-ers one month later. Two cranes were needed to lift these 30-tonne curved dis-sipators. Their handling requires consid-erable precision. Their installation, ex-cluding setting up of the equipment, lasts two weeks. (© C. Huret)

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New hydroelectric facility in Romanche-Gavet VGB PowerTech 4 l 2018

the offtakes (see F i g u r e 5 ) were in-stalled, followed by the four hydraulic plants with their accumulators and pipe-work. Pressure tests on the offtakes and dissipator parts were conducted at the same time. Delivered as of September 2017, the dissipators (see F i g u r e 6 ) were in-stalled in their respective pits.

Wall crossing

Base of the dissipator with silentbloc

Fig. 7. Cross-section of a dissipator in its pit. On the top, the wall crossing where stress can reach 168 tonnes. On the bottom, the base of the dissipator, where stress is around 30 tonnes, is mounted on a silentbloc to lower vibration levels. © D2FC (zoom to be extracted from attached JPEG)

Once the reinforced wells are built and the 276-m penstock installed in its tunnel, the bend joining up with the dispatcher could be concreted and all the welds between the different parts (manifolds and ring spools of the ball valves) will be completed.

Adapted civil engineeringReinforcement and concreting of the pits and walls of the building are calculated ac-cording to the great stress to which they will be subjected during the operation of the dissipators. The bottom slab is 1 m thick over 450 m2. 2,400 m3 of concrete surround the tanks and 320 tonnes of iron framework were needed for the whole building. To reduce potential vibrations on the inner wall of the building, crossed by offtakes of the dissipators, shock-absorbing devices were installed on an offtake flange. Like-wise, the bases of the dissipator in the pits were mounted on silentblocs to reduce vibrations. Stress linked to the weight of the operating dissipator is mainly vertical, at around 30 tonnes, while the stress on the wall crossing is 168 tonnes (see F i g -u r e   7 ). The bend of the penstock is concreted and the end of the dispatcher (manifold) is also held in a concrete block to ensure stability of the upstream part of the dissipators.

Priming tests

In early 2018, dry runs will be conducted on the hydraulic plants. Water testing of the whole dissipation system may only be conducted with the priming of the facility scheduled in 2019. Because they are indis-pensable to the safety of the facility, the dissipators will be among the first equip-ment to be water-tested, just after testing of the watertightness of the headrace and op-eration of the ball valves. l

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The electricity sector

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Flexibility and Storage

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International Journal for Electricity and Heat Generation

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